The present application claims priority under 35 U.S.C. xc2xa7119 of French Patent Application No 99 08398 filed Jun. 30, 1999; the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
This invention relates to a laser source fitted with a retroreflecting-dispersing system in the Littman-Metcalf configuration.
2. Discussion of Background Information
It is known to have retroreflecting-dispersing systems used in a manner to form one of the retroreflecting systems of a resonant laser cavity, in order to select spectrally one or some of the rays capable of being produced by the cavity.
By way of example, French patent FR-2.595.013 describes a monomode laser source of such type. The transmission wavelength is selected in the wide spectrum of an amplifier wave-guide with an external cavity comprising a retroreflecting dispersing system containing a diffraction grating. In the case of this prior art, a movement changing the angular orientation of the dispersing device enables varying the wavelength selected and transmitted by the laser source.
These laser sources implement a retroreflecting dispersing assembly in the so-called Littman-Metcalf configuration where the incident collimated beam delineates an angle xcex81 with the normal line of the grating. An additional reflector is placed with its normal line describing an angle xcex82 on the grating and the wavelength xcex which meets the condition xcex=xcfx81 (sin xcex81+sin xcex82) where xcfx81 is the step of the grating, is dispersed by the grating at an angle xcex82, then retroreflected on the reflector that is then perpendicular to the former. Finally, it is dispersed again in the grating on the way back and comes out under the input angle xcex81. The wavelength xcex is therefore selected by the cavity. It is possible to vary this wavelength xcex by varying the orientation of the grating-reflector assembly, i.e., by varying xcex1 or varying the orientation of the reflector only, i.e., by varying xcex82 or still by varying the orientation of the grating only, i.e., by varying xcex81 and 02 while keeping xcex81xe2x88x92xcex82 constant.
Such an external cavity laser source fitted with a retroreflecting dispersing system 2 in the Littman-Metcalf configuration is represented in FIG. 1 and may be described as comprising a first active arm 3 containing an amplifier guide 4, for example a semiconducting diode, a diffraction grating 5 and second passive arm 6 closed by a reflector 7.
The usual configuration of these sources uses the partially reflecting external face 8 of the diode 4 as an exit gate of the laser, and a total reflector then closes the second arm 6. This reflector can be a plane mirror or a dihedron, as illustrated in FIG. 2, ensuring one-dimension self-alignment of the cavity as described in French patent FR-2.595.013. Self-alignment is defined as the property of an optical system for which the properties of the outgoing luminous flux are minimally sensitive to the orientation or to the position of the system with respect to the incoming flux. Self-alignment can be performed on two dimensions, i.e., on all the planes parallel to the direction of the incoming beam or on a single dimension, i.e., on one of these planes only, the latter property being of special interest.
A collimation lens 9 formed by an optical system converging to the focus containing the internal face 10 of the amplifier wave guide 4, makes the beam emerging from this guide 4 parallel.
It has also been suggested to use the zero order of the grating 5 as an exit gate 8xe2x80x2 with a totally reflecting external face 8 of the amplifier guide 4. In both cases, the transmitted light comes directly from the amplifier guide and comprises a laser ray with high spectral fineness associated with a residual wide band continuous background, that is called amplified spontaneous emission (ASE). The residual ASE power may reach several percent of the ray power, which brings a limitation in the tuneable laser spectroscopic applications. A filter tuned on the emission wavelength should then be added at the output, but wavelength synchronization of the filter on the emission wavelength of the source is very tricky.
The purpose of the invention is to improve such a source in order to filter the residual ASE of the output luminous beam of the cavity directly without any significant power loss and by avoiding the use of an additional tuned filter.
To this effect, the invention relates to an external cavity laser source with integral filtering of the amplified spontaneous emission fitted with a retroreflecting dispersing system in the Littman-Metcalf configuration comprising a first active arm extending from a first end reflector up to the grating and containing an amplifier medium that produces a first luminous beam, whereas a second passive arm extends from the grating to a second end reflector, wherein the grating generates a second beam by diffraction of the first beam.
According to the invention:
the reflector closing the second passive arm is partially reflecting and enables extraction of a luminous flux and,
optical sources produce, from the second beam, a third beam translated and antiparallel to return to the grating which, by diffraction, forms a fourth beam translated and antiparallel to the first beam.
In different embodiments, each presenting specific advantages and capable of being combined according to numerous technically possible configurations:
the optical ASE filtering devices are placed in the path of the beam topped by the partially reflecting reflector of the second arm and sent back, after leaving the cavity, to the grating, in a direction parallel to that of the second arm; or
the optical devices are placed in the second arm of the source that is folded parallel to itself, goes through the grating again and ends on the partially reflecting reflector;
the laser source comprises a wave-guide forming the amplifier medium and whose external face, entirely reflecting, terminates the first arm;
the laser source is wavelength tuneable by rotation of the reflector of the second arm;
the reflector is a partially reflecting optical component that produces from an incident beam, two secondary beams, one transmitted, the other reflected, having a first plane face that is entirely reflecting and a second plane face that is partially reflecting; the second face is perpendicular to the first, a third plane face is entirely reflecting, whereby the first and third faces are situated on the same plane; this component ensures one-dimension self-alignment of the reflected beam with the incident beam, whereby the beams respectively transmitted and reflected are each composed of two semi-beams with matching wave fronts;
the partially reflecting optical component is such that the first and the second faces are the faces of the same first prism and that the third face is carried by a second prism carrying a fourth face in contact with the second face of the first prism; preferably, both these prisms have the same refraction index;
the partially reflecting optical component comprises a fifth face parallel to the third face, whereas the transmitted beam is sent back parallel in the same direction as the incident beam;
the partially reflecting optical component comprises a fifth face parallel to the second face, whereas the transmitted beam is sent back parallel to the incident beam and in reverse direction;
the partially reflecting optical component is such that the first and second prisms are fixed to one another and form an integral block whereby the orientations of their faces with respect to one another are controlled, regardless of the prism to which they belong;
the partially reflecting optical component is such that one of the second and fourth faces exhibits a partially reflecting treatment.
A laser source according to the invention comprises a first active arm extending from first end reflector up to a dispersing system grating and containing an amplifier medium that produces a first luminous beam. A second passive arm extends from the grating to a second end reflector, the grating generating a second beam by diffraction of the first beam. The second end reflector closes the second passive arm and is partially reflecting to enable extraction of a luminous flux. An optical component produces from the second beam, a third beam translated and antiparallel, to return to the grating which, by diffraction, forms a fourth beam translated and antiparallel to the first beam.
Further, according to the invention, the laser source comprises an external cavity laser source. The laser source further comprises integral filtering of an amplified spontaneous emission. The source is fitted with a retroreflecting dispersing system. The dispersing system is in a Littman-Metcalf configuration.
According to a further aspect of the invention, the optical component is placed in the path of a beam tapped by the second end reflector and sent back after leaving the cavity, to the grating, in a direction parallel to that of the second arm. The beam tapped by the second end reflector comprises the second beam. The optical component is placed in the second arm is folded parallel to itself, goes through the grating again and ends on a partially reflecting reflector. The partially reflecting reflector comprises the second end reflector. The source comprises a wave-guide forming the amplifier medium and whose internal face include an antiglare coating placed at the focus of a collimation lens and whose external face, entirely reflecting terminates the first arm. The source is wavelength tuneable.
According to another aspect of the invention, the second end reflector comprises a first plane face that is entirely reflecting, a second plane face that is partially reflecting, the second face being perpendicular to the first face, and a third plane face that is entirely reflecting, wherein the first and third face are located in the same plane to ensure one-dimension self-alignment of the reflected beam with the incident beam, whereby the transmitted and reflected beams each comprise two semi-beams with matching wave fronts. The first and the second face are faces of the same first prism and the third face is carried by a second prism carrying a fourth face in contact with the second face of the first prism. The second end reflector comprises a fifth face parallel to the third face, the transmitted beam being sent back parallel in the same direction as the incident beam.
Further yet, the second end reflector comprises a fifth face parallel to the second face, the transmitted beam being sent back parallel to the incident beam and in a reverse direction. The first and second prisms are fixed to one another and form an integral block whereby the orientations of their faces with respect to one another are controlled, regardless of the prism to which they belong. One of the second and fourth faces exhibits a partially reflecting treatment.
According to yet another aspect of the invention an external cavity wavelength tuneable laser source with integral filtering of an amplified spontaneous emission fitted with a retroreflecting dispersing system in a Littman-Metcalf configuration comprises a first active arm extending from a first end reflector up to a dispersing system grating and containing an amplifier medium that produces a first luminous beam, a second passive arm extending from the grating to a second end reflector, the grating generating a second beam by diffraction of the first beam. The second end reflector closes the second passive arm and is, partially reflecting to enable extraction of a luminous flux. An optical component produces from the second beam a third beam translated and antiparallel, to return to the grating which, by diffraction, forms a fourth beam translated and antiparallel to the first beam. The optical component is placed in the path of the second beam which is sent back after leaving the cavity to the grating, in a direction parallel to that of the second arm. The second end reflector comprises a first plane face that is entirely reflecting, a second plane face that is partially reflecting, the second face being perpendicular to the first face, and a third plane face that is entirely reflecting, wherein the first and third faces are located in the same plane to ensure one-dimension self-alignment of the reflected beam with the incident beam whereby the transmitted and reflected beams each comprise two semi-beams with matching wave fronts. The first and the second faces are faces of the same first prism, the third face being carried by a second prism carrying a fourth face in contact with the second face of the first prism. The second end reflector comprises a fifth face parallel to the third face, the transmitted beam being sent back parallel in the same direction as the incident beam. The first and second prisms are fixed to one another and form an integral block whereby the orientations of their faces with respect to one another are controlled, regardless of the prism to which they belong with one of the second and fourth faces exhibiting a partially reflecting treatment.